The semiconductor integrated circuit (IC) industry has experienced rapid growth. Technological advances in IC materials and design have produced generations of ICs where each new generation has smaller and more complex circuits than the previous generation. However, these advances have increased the complexity of processing and manufacturing ICs and, for these advances to be realized, similar developments in IC processing and manufacturing are needed. In the course of integrated circuit evolution, there has been an increase in functional density (the number of interconnected devices per chip area) and a decrease in geometric size (the smallest component or line that can be created using a fabrication process). This scaling-down process generally provides benefits by increasing production efficiency and lowering associated costs.
But scaling down presents challenges as well. For instance, as geometric size and critical dimension (CD) have decreased, it has become more difficult to obtain optimal photolithography results. Lithography tools have generally kept up with the scaling down trend by moving to light sources in the deep ultraviolet spectrum, but problems arise when smaller wavelength light is used to expose current photoresist materials. For example, when a lithography tool's depth of focus is smaller than the thickness of the layer of photoresist to be exposed, only a portion of the layer is actually exposed. Non-uniform exposure of a layer of photoresist may lead to non-uniform etch patterns and etch bias, ultimately degrading device performance.
Techniques to compensate for inadequate depth of focus have been devised and are generally adequate for their intended purpose, but they are not entirely satisfactory. For example, some suffer from problems such as unacceptable critical dimension variation (also known as “tiger skin”).